1. Field of the Invention
The present invention relates to a motor control apparatus for converting AC power supplied from an AC input side into DC power to be output and further for converting the DC power into AC power for driving a motor that is supplied to the motor, and in particular, to a motor control apparatus including at least two resistance discharge units.
2. Description of the Related Art
In motor control apparatuses for driving motors in machine tools, forging presses, injection molding machines, industrial machines, and robots, AC power input from an AC input side is temporarily converted into DC power, followed by further conversion into AC power, and this AC power is used as driving power of a motor provided in each drive axis. A motor control apparatus includes a rectifier for rectifying AC power supplied from an AC input side including a three-phase AC input power source to output DC power; and an inverter for performing power interconversion of DC power of a DC link and AC power that is driving power or regenerative power of a motor, the inverter being connected to the DC link that is a DC output side of the rectifier, in which a speed or torque of a motor connected to an AC output side of the inverter or a position of a rotor is controlled.
In recent years, from the viewpoint of a demand for saving energy, in a motor control apparatus, a regeneration-type rectifier capable of returning regenerative energy generated during deceleration of a motor to an AC input side is being frequently used.
However, in cases where a three-phase AC input power source disposed on the AC input side is a generator or power fails on the AC input side, regenerative energy is difficult to return to the AC input side. To cope with such situations, countermeasures are taken, in which a resistance discharge unit is added on a DC link between a rectifier and an inverter inside an existing motor control apparatus to consume regenerative energy generated during motor deceleration as thermal energy of a resistance (referred to also as a “discharge resistor”) inside the resistance discharge unit.
FIG. 15 is a view illustrating a configuration of a general motor control apparatus including a resistance discharge unit. As illustrated in FIG. 15, a motor control apparatus 401 includes a rectifier 411 for rectifying AC power supplied from an AC input side including a three-phase AC input power source 3 to output DC power; an inverter 412 for performing power interconversion of DC power of a DC link and AC power that is driving power or regenerative power of a motor 2, the inverter 412 being connected to the DC link that is a DC output side of the rectifier 411; and a resistance discharge unit 413 for consuming regenerative energy by conversion into thermal energy in a resistance, the resistance discharge unit 413 being connected to the DC link. In accordance with motor drive commands from a control apparatus (not illustrated), the inverter 412 converts DC power in the DC link and outputs AC power of a desired frequency for motor drive. Regenerative energy generated in the motor 2 during deceleration of the motor 2 is converted into DC power by the inverter 412, followed by further conversion into AC power by the rectifier 411 to be returned to the AC power input side including the three-phase AC input power source 3.
The resistance discharge unit 413 includes a resistance discharge part 421, a voltage detection part 422, and a discharge operation determination part 423.
Of these, the resistance discharge part 421 includes a resistance R; a switching device S for connecting the resistance R to a DC link between the rectifier 411 and the inverter 412 upon reception of commands (ON signals) for starting a resistance discharge operation from the discharge operation determination part 423 and for disconnecting the resistance R and the DC link upon reception of commands (OFF signals) for stopping the resistance discharge operation from the discharge operation determination part 423; and a reflux diode D.
Further, the voltage detection part 422 detects a voltage of a DC output side of the rectifier 411 (in other words, a voltage of a smoothing capacitor C). A detected DC voltage value is transmitted to the discharge operation determination part 423. The resistance discharge unit 413 is added on the DC link between the rectifier 411 and the inverter 412 in order to cope with the case where regenerative energy is difficult to return to the AC input side as described above. As a result, there occurs a case in which it is difficult to provide a communication interface between an existing device such as the rectifier 411 and the resistance discharge unit 413 to be added on or communication rate is low even with the communication interface. Therefore, inside the resistance discharge unit 413 to be added on, the voltage detection part 422 is provided for detecting a DC voltage in the DC link.
Further, the discharge operation determination part 423 produces commands (ON/OFF signals) for on/off-controlling the switching device S inside the resistance discharge part 421 in response to a DC voltage value detected by the voltage detection part 422.
The switching device S inside the resistance discharge part 421 is switched on or off in accordance with commands received from the discharge operation determination part 423. Thereby, the resistance R and the DC link are connected or this connection is cut off. Regenerative energy generated during deceleration of the motor 2 is converted into DC power by the inverter 412, whereby a DC voltage in the DC link (in other words, a voltage of the DC output side of the rectifier 411) increases and then upon exceeding a certain threshold value, the discharge operation determination part 423 outputs ON signals to the switching device S. As a result, the switching device S is switched on and then the resistance R is connected to the DC link, whereby the regenerative energy is consumed in the resistance R by conversion into thermal energy (resistance discharge operation start). The DC voltage in the DC link (in other words, the voltage of the DC output side of the rectifier 411) drops due to the consumption of this thermal energy, and then, upon falling below a certain threshold value, the discharge operation determination part 423 outputs OFF signals to the switching device S. As a result, the switching device S is switched off and the connection between the resistance R and the DC link is cut off, whereby the consumption of the regenerative energy in the resistance R stops (resistance discharge operation stop).
When the three-phase AC input power source on the AC input side is a generator or power fails on the AC input side, regenerative energy is difficult to return to the AC input side. When this fact is neglected, a DC voltage in the DC link exceeds voltage resistances of elements of the rectifier 411 and the inverter 412, resulting in apparatus breakage. The aforementioned discharge operation unit 413 is one unit to prevent this situation.
For example, Unexamined Japanese Patent Application Publication No. 2002-120973 discloses a technique, in which as a countermeasure against the case where power has failed on an AC input side, using a hoist for lifting an elevator and an inverter apparatus including a power consumption circuit, an elevator cargo is guided to a nearest floor with continuous braking of the elevator even upon failure on the AC input side.
Further, for example, Unexamined Japanese Patent Application Publication No. 2003-088144 discloses a technique, in which in an inverter control apparatus including a discharge resistance, a discharge control operation is executed only in cases where an inverter operation has stopped or a main power source has been cut off due to a power failure or other reasons to lower a main circuit DC bus voltage in a short period of time.
Further, for example, Unexamined Japanese Patent Application Publication No. 2006-262616 discloses a technique, in which in an inverter apparatus for consuming regenerative power from a motor using a discharge resistor disposed in a DC link portion between a rectifier and an inverter, a pre-charge contact is off-controlled upon detecting a short-circuit failure of a switching device to protect the discharge resistor from overheat and burnout due to fusing of a pre-charge registor.
Further, for example, Unexamined Japanese Patent Application Publication No. 2009-213200 discloses a technique, in which in a motor drive control system including a power regenerative unit for regenerating energy generated by a motor to a power source and a heat regenerative unit for performing conversion into heat, motor phase current is stabilized in a wide range of the number of motor rotations.
With respect to the aforementioned resistance discharge unit, it is necessary to determine a discharge capacity of the resistance discharge unit in accordance with a regenerative energy amount of a motor driven by a motor control apparatus. However, when countermeasures are intended for regenerative energy amounts of various types of motors, it is necessary to provide resistance discharge units having discharge capacities corresponding to these, resulting in that the model type of the resistance discharge units is increased and therewith, maintainability is degraded.